With a dramatic proliferation of information communications equipment, especially, personal compact equipment such as a mobile terminal, components of this equipment such as a memory device and a logic device are required to have higher performance such as higher integration, higher speed, and lower power consumption. In particular, a nonvolatile memory is considered to be an indispensable device in the ubiquitous age.
Even in the event of consumption or trouble of a power source, or line disconnection between a server and a network due to any failure, for example, the nonvolatile memory can protect personal important information. Increasing the storage density and capacity of the nonvolatile memory is becoming more important as a technique for substituting the nonvolatile memory for a hard disk or optical disk, which cannot substantially be made compact because of the presence of a movable part.
Further, recent mobile equipment is so designed as to suppress power consumption as low as possible by making an unnecessary circuit block in a standby state. In this respect, if a nonvolatile memory capable of serving both as a high-speed network memory and as a high-capacity storage memory can be realized, undue power consumption and memory can be eliminated. Further, a so-called instant-on function such that instantaneous starting is allowed at power-on can also become possible if a high-speed, high-capacity nonvolatile memory can be realized.
Examples of the nonvolatile memory include a flash memory using a semiconductor and an FRAM (Ferro electric Random Access Memory) using a ferroelectric material. However, the flash memory has a drawback such that the writing speed is as low as on the order of microseconds. On the other hand, as pointed out in the art, the FRAM has a problem such that the number of times of attainable rewriting is 1012 to 1014 and the durability is therefore low in comparison with a static random access memory or a dynamic random access memory. Another problem on the FRAM is that microforming of a ferroelectric capacitor is difficult.
Attention is being given to a magnetic memory device called an MRAM (Magnetic Random Access Memory) as a nonvolatile memory eliminating the above problems. The MRAM in early stages is based on a spin valve using an AMR (Anisotropic Magneto Resistive) effect reported in J. M. Daughton, “Thin Solid Films” Vol. 216 (1992), p. 162–168 or a GMR (Giant Magneto Resistance) effect reported in D. D. Tang et al., “IEDM Technical Digest” (1997), p. 995–997. However, such an early MRAM has a drawback such that the resistance of a memory cell as a load is as low as 10 Ω to 100 Ω, so that the power consumption per bit in reading is large to cause a difficulty of increasing the capacity.
In a TMR (Tunnel Magneto Resistance) effect reported in R. Meservey et al., “Pysics Reports” Vol. 238 (1994), p. 214–217, the resistance change rate is as low as 1% to 2%. However, as recently reported in T. Miyazaki et al., “J. Magnetism & Magnetic Material” Vol. 139 (1995), L231, a higher resistance change rate of about 20% has been obtained, and attention has been focused on the MRAM using the TMR effect.
The MRAM is simple in structure, so that high integration is easy to perform. Furthermore, recording is performed by rotation of magnetic moments, so that the number of times of attainable rewriting is expected to be large. In addition, the access time is also expected to be very short, and a high operation speed of 100 MHz has already been reported in R. Scheuerlein et al., “ISSCC Digest of Technical Papers” (February 2000), p. 128–129.
The MRAM using the TMR effect stores information by utilizing the fact that the tunnel resistance in an oxide film (tunnel oxide film) having a thickness of 0.5 nm to 5 nm sandwiched between ferromagnetic layers changes according to the directions of magnetization in these ferromagnetic layers. However, the tunnel resistance largely changes with the thickness of the tunnel oxide film. Accordingly, it is necessary to precisely uniform the thickness of the tunnel oxide film. Variations in thickness of the tunnel oxide film must be suppressed to about 3% to 5% depending on the scale of integration or the performance of a device.
The thickness of the tunnel oxide film varies not only in forming the tunnel oxide film, but also in a subsequent resist ashing step (K. Tsuji et al., “IEDM” (2001), p. 799) or in a sintering step using a forming gas. That is, reduction of the tunnel oxide film or oxidation of the ferromagnetic layers sandwiching the tunnel oxide film occurs because of the diffusion of hydrogen contained in an interlayer dielectric film or the diffusion of hydrogen or oxygen in the sintering step, thus resulting in variations in thickness of the tunnel oxide film. In the event that the thickness of the tunnel oxide film becomes too small, there arises a problem that the ferromagnetic layers may be short-circuited.